Area Forecast Discussion
Issued by NWS Great Falls, MT

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FXUS65 KTFX 180542
AFDTFX

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE GREAT FALLS MT
1035 PM MST Wed Dec 17 2014

Aviation Section Updated.

.UPDATE...
Latest observations show freezing fog, dense in areas, continuing
over most of the Hiline counties and redeveloping in parts of
Pondera and Teton counties. So have extended the freezing fog
advisory for eastern Glacier and Toole and added eastern Pondera,
Teton, Liberty, and Hill counties to the advisory that now goes
until 9am Thurs. Also adjusted the overnight lows in the fog areas
as several sites were at/below the forecast minimum temps. A weak
shortwave trof currently crossing western MT may help increase the
surface winds east of the Divide during the overnight hours to
help dissipate some of the fog, but confidence is only marginal
for that to occur.
Waranauskas

&&

.AVIATION...
UPDATED 0535Z.
Persistent freezing fog has started to erode at KCTB with downslope
mixing, transitioning to VFR conditions. Dense freezing fog has
developed further east along the highline including KHVR. Expect
this to continue with LIFR conditions through early morning. Patchy
fog is periodically moving across the airfield at KHLN, yielding
flight category changes on the order of tens of minutes. Otherwise,
VFR conditions prevail through the period under generally clear
skies. High cloud cover will increase Thursday as another weak
disturbance moves into western Montana. Nutter

&&

.PREV DISCUSSION...
/ISSUED 435 PM MST Wed Dec 17 2014/
Wednesday night through Friday...A rather quiet and cool end to
the work week is expected as no major systems are expected to move
over the area in this time frame. Temperatures will remain near
seasonal values as weak shortwave toughing on Thursday will be
followed by a week shortwave ridge on Friday. The biggest impact
during this time period will be the persistent freezing fog over
parts of Central and North Central Montana. Have extended the
advisory currently in effect until 9 pm for areas around Cut Bank
and Shelby. Westerly winds should develop around 6 pm and
eventual scour out the fog. However, there is a chance that the
nocturnal boundary layer will strengthen the inversion and will
limit any potential for the west winds to move the fog out of the
area until Thursday when a weak shortwave moves through the area.
Widespread moisture is not expected at this time with this
disturbance...however some isolated mountain snow showers are
possible. Suk

Friday Night through Wednesday...A weak shortwave trough will move
through Montana Friday night into Saturday, bringing just a chance
of snow to the mountains. Forecast models continue to indicate
that the pattern will then change for Saturday night through
Tuesday. An upper level high pressure ridge will strengthen off
the California coast and a low pressure trough will deepen from
Hudson Bay into the Great Lakes. This will result in Montana being
under a strong northwesterly flow aloft. The models indicate that
the strongest winds aloft will occur Sunday into Sunday night with
the passage of a disturbance embedded in the flow. However, the
models differ on just how strong these winds will be. The ECMWF
only forecasts winds of 45 to 55 kts at 700 mb, while the GFS
generally has 60 to 70 kt winds. Have therefore used a compromise
solution between these two models with regards to surface wind
strength. Despite this difference, this set-up will bring gusty
westerly downslope winds to much of the area, but especially over
the plains of north central Montana. In fact, some areas along the
Rocky Mountain Front could approach High Wind criteria (40 mph
sustained/58 mph gusts). Confidence remains moderate that High
Wind highlights may be needed for this event. The only concern is
that the northwesterly wind direction is not as favorable for
widespread High Winds. There is also significant moisture with
this disturbance, so have continued to spread the chance for
precipitation out onto the plains. Winds aloft will decrease
somewhat after the passage of the disturbance, but the
northwesterly flow aloft will remain fairly moist, so have kept
the chance for precipitation above climatological normals across
the area into Tuesday. A potentially significant weather system
originating in the Gulf of Alaska is then forecast to move into
Montana Tuesday into Wednesday, bringing a very good chance of
precipitation. Because this feature is near the end of the period,
exact timing and types of precipitation are very uncertain.
Regardless, this system will need to be monitored for potential
winter weather highlights, especially considering that it would be
moving into the area during peak holiday travel time. The breezy
winds through Monday will keep temperatures around 10 degrees
above normal. The warmest day of the period appears to be Monday,
when most lower elevation locations should get well into the 40s.
However, with the system moving into the area at the end of the
period, temperatures will likely fall back to near or slightly
below seasonal averages by Wednesday. Coulston

&&

.PRELIMINARY POINT TEMPS/POPS...
GTF  25  40  26  40 /  10  10  10  10
CTB  12  39  24  39 /  10  10  10  10
HLN  19  34  19  35 /  10  20  10  20
BZN  14  30  15  32 /  10  10  10  20
WEY  17  32  19  33 /  20  20  30  40
DLN  18  34  19  37 /  10  20  10  20
HVR  16  37  22  39 /  10  10  10  10
LWT  23  43  24  43 /  10  10  10  10

&&

.TFX WATCHES/WARNINGS/ADVISORIES...
FREEZING FOG ADVISORY until 9 AM MST Thursday Eastern Glacier...
Eastern Pondera...Eastern Teton...Hill...Liberty...Southern
Rocky Mountain Front...Toole.

&&

$$

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